When a vibration-sensitive module such as an electronic control unit is mounted on a vibrational base such as an internal combustion engine, elastomeric bushings are interposed between the module and the base to dampen the amount of vibrational energy transmitted from the base to the module. If too much vibrational energy is transmitted to the module, electronic components and solder joints in the module can suffer fatigue failures that degrade the performance of the module or render it inoperative.
FIG. 1 depicts a prior art arrangement for mounting an electronic module on a vibrational base 10. The module housing 12 is typically a cast or stamped metal part, but in some cases may be formed of plastic. In any case, the housing 12 has several integrally formed mounting plates or flanges, one of which is designated in FIG. 1 by the reference numeral 12a. The flange 12a is provided with an internal through-hole 12b for receiving a bushing assembly 14, and a mounting bolt 16 passing through the center of the bushing assembly 14 is threaded into a tapped opening in the vibrational base 10. The bushing assembly 14 includes upper and lower annular rings of elastomeric material (i.e., bushings) 18 and 20, an upper washer 22 disposed between the upper bushing 18 and the head 16a of bolt 16, a lower washer 24 disposed between the lower bushing 20 and the vibrational base 10, and a tubular sleeve 26 disposed about the bolt 16 between the upper and lower washers 22 and 24 that limits the bolt travel and determines the preset compression force applied to bushings 18 and 20. As seen in FIG. 1, the faces of bushings 18 and 20 that seat on the flange 12a are undercut as indicated by the reference numerals 18a and 20a, allowing the bushings 18 and 20 to partially extend into the flange through-hole 12b. This centers the bushings 18 and 20 in the flange through-hole 12b during installation and mounting, and fills part of the through-hole 12b with bushing material that acts to absorb vibrational movement of the base 10 in a direction transverse to bolt 16.
The elastomeric material of bushings 18 and 20 is designed to absorb vibrational movements of the base 10. However, the configuration of bushing assembly 14 places most of the elastomeric material between the flange 12a and the upper and lower washers 22 and 24, and only a small amount of the elastomeric material (i.e., the bushing portions 18a and 20a) within the flange through-hole 12b. Consequently, the bushing assembly 14 exhibits good absorption of vibrational impulses parallel to the longitudinal axis of bolt 16 (referred to herein as z-axis impulses), but only limited absorption of impulses in directions transverse to the z-axis before the inner ring of elastomeric material within the through-hole 12b locally collapses or bottoms-out. When a transverse impulse collapses the bushing material between the flange 12a and the sleeve 26, the bushing assembly 14 can no longer isolate the module 12, and the force transmitted to module 12 from the base 10 rises sharply. Even in a best-case scenario where the vibrational force impulse is predominantly along the z-axis, the elastomeric material of bushings 18 and 20 resonantly couple the vibrational movement into the transverse plane, resulting in transmitted vibrational vectors that can even exceed the z-axis force impulse. Of course vibrational force impulses are rarely unidirectional in nature, and the bushing assembly 14 is frequently incapable of adequately isolating module 12 in the transverse plane.
In view of the above, what is needed is an improved but cost-effective mounting arrangement that more effectively isolates a vibration sensitive module from both z-axis and transverse plane movements of a vibrational base.